Monolayer films of semiconducting metal dichalcogenides, methods of making same, and uses of same

ABSTRACT

Metal-chalcogenide films disposed on a substrate comprising at least one monolayer (e.g., 1 to 10 monolayers) of a metal-chalcogenide. The films can be continuous (e.g., structurally and/or electrically continuous) over 80% or greater of the substrate that is covered by the film. The films can be made by methods based on low metal precursor concentration relative to the concentration of chalcogenide precursor. The methods can be carried out at low water concentration. The films can be used in devices (e.g., electrical devices and electronic devices).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/130,407, filed Apr. 15, 2016, which claims priority to U.S.Provisional Patent application No. 62/148,387, filed Apr. 16, 2015, thedisclosures of which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under contract nos.FA2386-13-1-4118 and FA9550-11-1-0033 awarded by the Air Force Office ofScientific Research and contract nos. 1120296 and 0335765 awarded byNational Science Foundation. The government has certain rights in theinvention.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to metal-chalcogenide films.More particularly, the present disclosure relates to semiconductingmetal-chalcogenide films.

SUMMARY OF THE DISCLOSURE

In an aspect, the present disclosure provides metal-chalcogenide films(also referred to herein as metal-dichalcogenide films). The films canhave one or more metal and/or one or more chalcogenide. The filmscomprise one or more monolayers of a metal-chalcogenide disposed on(i.e., in contact with) a substrate. The films exhibit desirable spatialuniformity and/or electrical performance. The films are crystalline. Thefilms comprise 1 to 10 metal-chalcogenide monolayers.

In an aspect, the present disclosure provides a method of making themetal-chalcogenide films. The methods are based on low metal precursorconcentration relative to the concentration of chalcogenide precursor.The methods are based on a layer-by-layer growth mode. In an embodiment,the films are formed by a method of the present disclosure.

In an embodiment, a method for making the metal-chalcogenide films on asubstrate comprise: contacting a metal precursor, a chalcogenideprecursor, a reducing gas (e.g., hydrogen gas), and a substrate in areactor such that the metal-chalcogenide film is formed on thesubstrate. The precursors are present at low pressure in the reactor andin the gas phase. The films are not formed by sublimation of aprecursor.

In an aspect, the present disclosure provides uses of themetal-chalcogenide films of the present disclosure. The films can beused in a variety of devices. In an embodiment, a device (e.g., anelectronic device) comprises one or more metal-chalcogenide film of thepresent disclosure. The films can be used in, for example, transistors,P—N junctions, logic circuits, analog circuits. Examples of devicesinclude, but are not limited to, lasers, photo-diodes, opticalmodulators, piezoelectric devices, memory devices, and thin filmtransistor on transparent substrates. The films can provide afunctionality of a device. For example, the films can be used intransistors, P—N junctions, logic circuits, and analog circuits indevices such as, but not limit to lasers, photo-diodes, opticalmodulators, piezoelectric devices, memory devices, and thin filmtransistors. In an embodiment, an optical fiber comprises one or moremetal-chalcogenide film of the present disclosure. For example, theoptical fiber can be used in an optical modulator.

BACKGROUND OF THE DISCLOSURE

The large scale growth of semiconducting thin films is the basis ofmodern electronics and optoelectronics. Reducing film thickness to theultimate limit of the atomic, sub-nanometer length scale, a difficultlimit for traditional semiconductors (e.g., Si and GaAs), would bringwide benefits for applications in ultrathin and flexible electronics,photovoltaics and display technology. For this, transition metaldichalcogenides (TMDs), which can form stable three-atom-thickmonolayers (MLs), provide semiconducting materials with high electricalcarrier mobility, and their large-scale growth on insulating substrateswould enable batch fabrication of atomically-thin high-performancetransistors and photodetectors on a technologically relevant scalewithout film transfer. In addition, their unique electronic bandstructures provide novel ways to enhance the functionalities of suchdevices, including the large excitonic effect, bandgap modulation,indirect-to-direct bandgap transition, piezoelectricity andvalleytronics. However, the large-scale growth of ML TMD films withspatial homogeneity and high electrical performance remains an unsolvedchallenge.

Existing growth methods for large-scale ML TMDs have so far producedmaterials with limited spatial uniformity and electrical performance.For instance, the sulphurization of metal or metal compounds onlyprovides control over the average layer number, producingspatially-inhomogeneous mixtures of mono-, multi-layer and no-growthregions. While chemical vapour deposition (CVD) based on solid-phaseprecursors (e.g. MoO₃, MoCl₅, or WO₃) has demonstrated better thicknesscontrol over large scale, the electrical performance of the resultingmaterial, which is often reported from a small number of devices inselected areas, fails to show spatially uniform high carrier mobility.

BRIEF DESCRIPTION OF THE FIGURES

For a fuller understanding of the nature and objects of the disclosure,reference should be made to the following detailed description taken inconjunction with the accompanying figures.

FIG. 1. Wafer scale ML TMD films. a-b, Photo of ML MoS₂ (a) and WS₂ (b)film grown on 4-inch fused silica substrate with schematics for theirrespective atomic structure. Left half shows bare fused silica substratefor comparison. c, Photo of patterned ML MoS₂ film on 4-inch SiO₂/Siwafer (the darker areas are covered by MoS₂). d, Optical absorptionspectra of MOCVD grown ML MoS₂ and WS₂ films in the photon energy rangefrom 1.6 to 2.7 eV. e, Raman spectra of as-grown ML MoS₂ and WS₂,normalized to the silicon peak intensity. f, Normalizedphotoluminescence (PL) spectra of as-grown ML MoS₂ and WS₂. The peakpositions are consistent with those seen from exfoliated samples,denoted by diamonds, same for d-f. g, SEM image and PL image (bottominset, at 1.9 eV) of ML MoS₂ membranes suspended over SiN TEM grid with2-μm-sized holes (schematic of suspended film in the top inset). Scalebar, 10 μm. h-i, Optical images (normalized to the bare substrateregion) and PL images (inset, at 1.9 eV for MoS₂ and 2.0 eV for WS₂) ofpatterned ML MoS₂ and WS₂, respectively on SiO₂, taken from the waferscale patterned films. Scale bar, 10 μm.

FIG. 2. MOCVD growth of continuous ML MoS₂ film. a, Schematic of ourMOCVD growth setup. Precursors are introduced to the growth setup usingindividual mass flow controllers (MFCs). Dark grey=Mo or W atom, lightgrey=S, white=carbonyl or ethyl ligands. b, Optical images ofMOCVD-grown MoS₂ at different growth times, where t₀ is the optimalgrowth time for full ML coverage. Scale bar, 10 μm. c, Coverage ratiofor ML (θ_(1L)) and multi-layer region (θ_(≥2L)) as a function of growthtime. d, Grain size variation of ML MoS₂ depending on the hydrogen flowrate from left to right; 5 sccm (SEM image shown), 20 sccm (SEM) and 200sccm (TEM). e, False-colour DF-TEM image showing a continuous ML MoS₂film. Scale bar, 1 μm, ADF-STEM image of a laterally-stitched grainboundary in a ML MoS₂ film, with grey and white dots representing the Moand S atoms, respectively. Scale bar, 1 nm.

FIG. 3. Electrical characterization and batch fabrication of ML TMDFETs. a, Gate-dependent sheet conductance (σ_(□)) of ML MoS₂ FETsmeasured with different L, the channel length (curves shifted from thebottom for clarity). Inset: optical image of the device, scale bar 10μm. b, Field effect mobility (μ_(FE)) measured from five MoS₂ FETsfabricated at random locations with different L. Data from previousresults for CVD-grown and exfoliated samples are shown for comparison(stars indicating their medians). c, Temperature dependence of μ_(FE)measured from the device in FIG. 3a , and from a previous report onexfoliated samples, both showing the phonon limited intrinsic transport.d, Top gate (V_(TG)) dependent σ_(□) for dual-gate ML MoS₂ FET (deviceshown in upper inset). Lower inset: V_(TG)-dependent I_(SD)-V_(SD)curves showing current saturation and ohmic electrode contact. Scalebar, 10 μm. e, Gate-dependent σ_(□) of a ML WS₂ FET showing μ_(FE)=18cm²/Vs. Inset: V_(TG)-dependent I_(SD)-V_(SD) curves showing currentsaturation and ohmic electrode contact. f, (Left) Batch fabricated 8,100MoS₂ FET devices on a 4-inch SiO₂/Si wafer. f1, Zoom-in image of onesquare containing 100 devices. f2, f3, Corresponding colour map of σ_(□)at gate bias V_(BG)=50V and −50V respectively, with the black block inf2 representing the only non-conducting device. g, Histogram of on- andoff-state σ_(□) of 100 dual-gate FETs showing a median on-off ratio of10⁶ and a high on-state conductivity. All measurements were performed atroom temperatures except for c.

FIG. 4. Multi-stacking of MoS₂/SiO₂ structure. a, Schematics (left) andoptical image (right) of single, double and triple stacking ofML-MoS₂/SiO₂. b, Optical absorption spectra for single, double andtriple stacks, respectively (normalized spectra shown in the inset). c,Schematic for fabrication of MoS₂ device/SiO₂ stacking using alternatingMOCVD growth, device fabrication with photolithography, and SiO₂deposition. See main text for details. d, False-colour SEM image of MoS₂FET arrays on 1^(st) (bottom) and 2^(nd) (top) layer (zoomed-in image ofa pair of devices shown in the inset, scale bar 50 μm). e, I_(SD)-V_(SD)curves measured from two neighbouring devices on 1^(st) and 2^(nd)layer, respectively, both showing the n-type conductance switching.

FIG. 5. Raman spectra for MoS₂ (a) and WS₂ (b) respectively, taken atdifferent locations marked on the corresponding fused silica wafer.

FIG. 6. XPS spectra of a, Mo 3d 3/2, 5/2 and S 2s state for MoS₂ grownby our method (light grey) and bulk MoS₂ single crystal (grey), wherethe peak position and FWHM are almost identical. b, C is for MoS₂ grownby our method (light grey), bare SiO₂/Si substrate after solventcleaning (grey) and bulk single crystal (darker grey), where all threesample show similar peak area of C 1s, which means our films do notcontain significant carbon residue after MOCVD process (curves shiftedfrom the bottom for clarity).

FIG. 7. Optical reflection, PL, SEM images of MOCVD-grown MoS₂ atdifferent growth times, where t₀ is the optimal growth time for full MLcoverage.

FIG. 8. Schematics of growth (step I) and re-growth (step II) of MoS₂film and corresponding optical reflection, PL, DF-TEM images.

FIG. 9. Normalized intensity of residual gas signal for Mo(CO)_(x) andC_(x)H_(y)S depending on temperature. Each dot corresponds to atemperature, as denoted in the figure.

FIG. 10. Morphology change of MOCVD-grown MoS₂ depending on the growthparameters. In order to show grain size clearly, we intentionally growpartially covered MoS₂. a, salt (desiccant) dependence of grain size. b,DES flow rate dependence of grain size. c, high Mo vapour concentrationenvironment, where a mixture of monolayer, multilayer and no-growthregions exist.

FIG. 11. False-colour DF-TEM image of MoS₂ grown at two locations 8 cmapart, where the identical grain size and nucleation density suggeststhe homogeneous nucleation over the whole growth area. Scale bar, 100nm.

FIG. 12. a, A polar plot of the electron diffraction intensity(diffraction map shown in the inset) measured from a large area ML MoS₂(inset), indicating a uniform angular distribution of the MoS₂ crystalorientation. It is generated by averaging the diffraction intensity fromthe three equivalent angular domains, each spanning 120 degrees. b, Ahistogram of intergrain rotation angles measured from all grainboundaries found in the ML MoS₂ sample shown in FIG. 2e , suggesting nopreferred intergrain tilt angle. For this, the crystal orientation forevery MoS₂ grain was first obtained from five DF-TEM images taken withdifferent objective aperture locations (each centred at 0°, 12°, 24°,36° and 48°, respectively). In each DF-TEM image taken at an objectiveaperture location θ, we assign the crystal orientation of θ (forbrighter regions; from aligned Mo sub-lattice) or θ-60° (for less brightregions; from aligned S sub-lattice). The intergrain rotation angleswere extracted using these assignments and range between −60° to 60°(with ±6° error).

FIG. 13. a, ADF-STEM images of laterally-stitched grain boundaries in aML MoS₂ film. The sub nanometre holes come from electron beam radiationdamage, and the clouds are surface contaminations generated during thetransfer process. b, High quality ADF-STEM image of ML MoS₂ film and c,corresponding line profile of intensity. The image intensity is roughlyproportional to Z_(γ), where Z is the atomic number, and 1.3<γ<2.

FIG. 14. Gate-dependent σ_(□) of four multiple-electrode ML MoS₂ FETs(same geometry as shown in FIG. 3a inset) separated by up to 3.3 mm.

FIG. 15. a, Transfer curves measured from MoS₂ of grain size 3 μm withdifferent channel lengths. b, Field effect mobility (μ_(FE)) ofdifferent channel lengths extracted from (a). c, Temperature dependenceof μ_(FE) measured from MoS₂ film of grain size 3 μm, with differentchannel lengths, which show the same dependence as shown in FIG. 3 c.

FIG. 16. Gate-dependent two-terminal σ_(□) of four additional ML WS₂FETs, of which the extracted mobilities are 14.3, 14.1, 11.6 and 10.9cm²/Vs, respectively.

FIG. 17. Histograms of threshold voltage of MoS₂ FETs with grain size0.4, 1.0 and 2.6 μm, respectively.

FIG. 18. Statistics for field effect mobility of MoS₂ FETs taken fromthe devices in FIG. 3 g.

FIG. 19. Gate-dependent Go for ML MoS₂ FET on 1^(st) and 2^(nd) layer inthe multi-stacked device structure. The total gate oxide (SiO₂)thickness for 1^(st) and 2^(nd) layer devices are 285 nm and 785 nm,respectively.

FIG. 20. a-c, Raman spectra for MoS₂ grown on Al₂O₃, SiN and HfO₂covered Si, respectively. d-e, σ_(□)-V_(BG) curves for ML MoS₂ FET onAl₂O₃/Si and HfO₂/Si, respectively. The measured dielectric constant forAl₂O₃ and HfO₂ is 6.0 and 15.5, respectively. These MoS₂ films weregrown under the same conditions developed for SiO₂ substrates asdescribed in Methods, without any further optimization.

FIG. 21. a, The effect of measuring in vacuum and annealing on thedevice's electrical performance (the vacuum and annealing n-dope theMoS₂ devices). b, The transfer property of same device in ambient beforeand after HfO₂ encapsulation (encapsulation n-dopes MoS₂).

DETAILED DESCRIPTION OF THE DISCLOSURE

The present disclosure provides metal-chalcogenide films comprising oneor more monolayers of a metal-chalcogenide on a substrate. Also providedare methods of making the films and using the films.

Disclosed is, for example, high-mobility, 4-inch wafer-scale films of MLmolybdenum disulphide (MoS₂) and tungsten disulphide (WS₂), directlygrown on insulating SiO₂ substrates, with desirable spatial homogeneityover the entire films. The films are grown using a novel metal-organicchemical vapour deposition (MOCVD) technique, and show desirableelectrical performance, including an electron mobility of 30 cm²/Vs atroom temperature and 114 cm²/Vs at 90 K for MoS₂, with little positionor channel-length dependence. Using these films, the wafer-scale batchfabrication of high-performance ML MoS₂ field effect transistors (FETs)was demonstrated with a 99% device yield and the multi-level fabricationof vertically-stacked transistor devices for three-dimensionalcircuitry. This work represents an important step toward the realizationof atomically-thin integrated circuitry.

In an aspect, the present disclosure provides metal-chalcogenide films.The films can also be referred to as metal-dichalcogenide films. Thefilms can have one or more metal and/or one or more chalcogenide. Themetal is Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Nb, Mo, W, or a combinationthereof. For example, the films are chromium-, titanium-, molybdenum-,or tungsten-chalcogenide (e.g., sulfide, selenide, telluride, or acombination thereof) films. The films comprise one or more monolayers ofa metal-chalcogenide disposed on (i.e., in contact with) a substrate.The films exhibit desirable spatial uniformity and/or electricalperformance. The films can be made by a method disclosed herein.Accordingly, in an example, a film is made by a method disclosed herein.

The films are crystalline. The films comprise 1 to 10 metal-chalcogenidemonolayers. In various embodiments, the film comprises 1, 2, 3, 4, 5, 6,7, 8, 9, 10, monolayers. The films have a grain size of, for example,100 nm to 100 microns, including all nm values and ranges therebetween.The grains are laterally connected at the grain boundaries. Little or nograin boundary resistance is observable. The grain boundary resistancecan be measured by methods known in the art. For example, example grainboundary resistance is evaluated by fabricating electronic device acrossthe grain boundary, which is identified by transmission electronmicroscope imaging, and measuring the total conductance.

The films can have a wide range of area and form factors. The area andform factor of the film is based on the application and/or thefabrication reactor. For example, the films disposed on a discrete(i.e., not continuous) substrate have an area of 100 square microns toabout 50,700 square centimeters (254 cm diameter circular substrate).The films are conformal. The films may be continuous. In an embodiment,the substrate is a 4 inch diameter substrate.

The films provide at least a monolayer thick film across substantiallyall of the substrate (e.g., the portion(s) of the substrate that aredesired to be coated with the film). For example, the films provide atleast a monolayer thick across 80% or greater, 90% or greater, 95% orgreater, 99% or greater of the substrate that is covered by the film. Inanother example, the films are a monolayer thick across 100% of thesubstrate that is covered by the film. In an embodiment, the filmsprovide a 1 to 10 monolayer thick film across 80% or greater, 90% orgreater, 95% or greater, 99% or greater of the substrate that is coveredby the film. In another embodiment, the films are 1 to 10 monolayersthick across 100% of the substrate that is covered by the film. Invarious embodiments, the monolayer or monolayers are structurally and/orelectrically continuous across 80% or greater, 90% or greater, 95% orgreater, 99% or greater of the substrate that is covered by the film.For example, depending on growth time, film coverage (θ=1 is monolayer)can be changed from 0<θ≤1 (e.g., as shown in FIG. 2b and c ) and nodefects are observed (e.g., the defect level is under instrumentresolution, for example, according to XPS (e.g., FIG. 6) and STEM (FIG.2F)) when film coverage θ=1.

The films have desirable characteristics. For example, the films have adesirable level of defects (e.g., less than 10 ppm). Examples of defectsinclude grain boundaries and atom vacancies. For example, the films havedesirable mobility (e.g., at least 50 cm²V⁻¹s⁻¹ at room temperature andat least 130 cm²V⁻¹s⁻¹ at 90 K).

A variety of substrates can be used. The substrates may be planar ornon-planar. The substrate may be crystalline or amorphous. The substratemay be a fiber. The substrate may be continuous (i.e., a roll).

Examples of suitable substrates include silicon (e.g., with a nativesilicon oxide layer or silicon dioxide layer (e.g., PECVD or evaporatedsilicon dioxide layer)), quartz, fused silica, mica, silicon nitride,boron nitride, alumina, and hafnia. Suitable substrates are commerciallyavailable or can be fabricated by methods known in the art.

In an aspect, the present disclosure provides a method of making themetal-chalcogenide films. The methods are based on low metal precursorconcentration relative to the concentration of chalcogenide precursor.The methods are based on a layer-by-layer growth mode. In an embodiment,the films are formed by a method of the present disclosure.

In an embodiment, a method for making the metal-chalcogenide films on asubstrate comprise: contacting a metal precursor, a chalcogenideprecursor, a reducing gas (e.g., hydrogen gas), and a substrate in areactor such that the metal-chalcogenide film is formed on thesubstrate. The precursors are present at low pressure in the reactor andin the gas phase. The films are not formed by sublimation of aprecursor.

The metal precursor provides a source of metal for film formation.Examples of suitable metal precursors include metal carbonyl compounds.Examples of suitable Mo precursors include Mo(CO)₆, C₂₂H₂₂Mo₂O₆,C₁₆H₁₀Mo₂O₆, C₁₀H₁₀Cl₂Mo, C₁₁H₈MoO₄, and combinations thereof. Examplesof suitable W precursors include W(CO)₆, C₁₈H₂₆I₂W, (C₄H₉NH)₂W(C₄H₉N)₂,((CH₃)₃CN)₂W(N(CH₃)₂)₂, ((CH₃)₃CN)₂W(N(CH₃)₂)₂, C₁₀ H₁₀Cl₂W, C₁₀H₁₂W,(C₅H₄CH(CH₃)₂)₂WH₂, C₈H₆O₃W, C₁₂H₁₂O₄W, (NH₃)₃W(CO), and combinationsthereof. Examples of suitable metal precursors include theaforementioned metal precursors where the metal is Sc, Ti, V, Cr, Mn,Fe, Co, Ni, Nb, or a combination thereof.

The chalcogenide precursor provides a source of chalcogenide (e.g.,sulfide, selenide, or telluride) for film formation. In an embodiment,the chalcogenide precursor has one of the following structures:

(C_(x)H_(y))_(z)S, (C_(x)H_(y))_(z)Se, or (C_(x)H_(y))_(z)Te, where1≤x≤10, 1≤y≤20, 0≤z≤2, x and y are integers, z is an integer or fractionof an integer.

Examples of suitable chalcogenide precursors include alkyl chalcogenideprecursors (e.g., dialkylsulfide precursors, dialkylselenide precursors,and dialkyltelluride precursors). For example, the alkyl groups on theprecursors are individually selected from methyl groups and ethylgroups. Examples of suitable precursors include dimethylsulfide,dimethylselenide, dimethyltelluride, diethyl sulfide, diethylselenide,diethyltelluride, methylethylsulfide, methylethylselenide, andmethylethyltelluride.

Additional examples of metal precursors and chalcogenide precursorsinclude:

E_(g) (eV) Metal Precursors Chalcogenide Precursors MoS₂ 1.8~1.9 Mo(CO)₆(C₂H₅)₂S WS₂ 1.9~2.0 W(CO)₆ (C₂H₅)₂S MoSe₂ 1.5~1.6 Mo(CO)₆ (C₂H₅)₂Se,(CH₃)₂Se WSe₂ 1.7 W(CO)₆ (C₂H₅)₂Se, (CH₃)₂Se MoTe₂ 1.1 Mo(CO)₆(C₂H₅)₂Te, (CH₃)₂Te WTe₂ 1.1~1.2 W(CO)₆ (C₂H₅)₂Te, (CH₃)₂Te CrS₂ 1.1Cr(CO)₆ (C₂H₅)₂S CrSe₂  0.86 Cr(CO)₆ (C₂H₅)₂Se, (CH₃)₂Se CrTe₂  0.60Cr(CO)₆ (C₂H₅)₂Te, (CH₃)₂Te NiS₂  0.51 Ni(C₅H₅)₂ (C₂H₅)₂S TiS₂ MetallicTiCl₄, Ti[OCH(CH₃)₂]₄ (C₂H₅)₂S TaS₂ Metallic (CH₃CH₂O)₅Ta (C₂H₅)₂S NbSe₂Metallic (CH₃CH₂O)₅Nb (C₂H₅)₂Se, (CH₃)₂Se

The precursors can be present in a carrier gas. For example, the carriergas is argon, nitrogen or other inert gas. Without intending to be boundby any particular theory, it is considered that use of argon providesdesirable film uniformity.

The metal precursor is present (e.g., present in the reactor) at, forexample, 1×10⁻⁶ Torr to 1×10⁻² Torr, including all integer Torr valuesand ranges therebetween. The chalcogenide precursor is present at 1×10⁻⁵Torr to 1×10⁻¹ Torr, including all integer Torr values and rangestherebetween. It is desirable that the ratio of metal precursor:chalcogenide precursor be 1:10 to 1:1000, including all integer valuesand ranges therebetween.

The reducing gas is present at, for example, 1×10⁻⁴ to 10 Torr,including all integer Torr values and ranges therebetween. Examples ofsuitable reducing gases include hydrogen gas.

It is considered that the presence of water during the fabricationprocess negatively effects the film forming process. Accordingly, it isdesirable that the method is carried out at low water concentration(less than 1×10⁻² Torr). In an embodiment, the method is carried out inthe presence of a desiccant. For example, the desiccant is present in areaction chamber where the method is carried out. Examples of suitabledesiccants include NaCl, KCl, or NaBr.

The film forming reaction is carried out at a range of temperatures. Forexample, the film forming reaction is carried out at 300° C. to 700° C.,including all integer ° C. values and ranges therebetween. Filmstructure (e.g., grain size, morphology, and number of layers) can varydepending on the temperature at which the film forming reaction iscarried out. Typically, higher temperatures generate larger grain sizeand more layers.

The steps of the methods described in the various embodiments andexamples disclosed herein are sufficient to make a metal-chalcogenidefilm of the present disclosure. Thus, in an embodiment, a methodconsists essentially of a combination of the steps of the methoddisclosed herein. In another embodiment, a method consists of suchsteps.

In an aspect, the present disclosure provides uses of themetal-chalcogenide films of the present disclosure. The films can beused in a variety of devices.

In an embodiment, a device (e.g., an electronic device) comprises one ormore metal-chalcogenide film of the present disclosure. The films can beused in, for example, transistors, P—N junctions, logic circuits, analogcircuits. Examples of devices include, but are not limited to, lasers,photo-diodes, optical modulators, piezoelectric devices, memory devices,and thin film transistor on transparent substrates. The films canprovide a functionality of a device. For example, the films can be usedin transistors, P—N junctions, logic circuits, and analog circuits indevices such as, but not limit to lasers, photo-diodes, opticalmodulators, piezoelectric devices, memory devices, and thin filmtransistors.

In an embodiment, an optical fiber comprises the metal-chalcogenidefilms of the present disclosure. For example, the optical fiber can beused in an optical modulator.

In the following Statements, various examples of the compositions,methods, and devices of the present disclosure are described:

-   1. A metal-chalcogenide film disposed on a substrate, the film    comprising at least one (e.g., one) monolayer of a    metal-chalcogenide.-   2. A metal-chalcogenide film of Statement 1, where the metal is Sc,    Ti, V, Cr, Mn, Fe, Co, Ni, Nb, Mo, W, or a combination thereof.-   3. A metal-chalcogenide film of any one of Statements 1 or 2, where    the chalcogenide is S (sulfide), Se (selenide), Te (telluride), or a    combination thereof.-   4. A metal-chalcogenide film of any one of the preceding Statements,    wherein the film comprises (or further comprises) 2 to 10 monolayers    of a metal chalcogenide.-   5. A metal-chalcogenide film of any one of the preceding Statements,    where the film is continuous (e.g., continuous (e.g., structurally    and/or electrically continuous) over, for example, 80% or greater,    90% or greater, 95% or greater, or 100% of the substrate that is    covered by the film).-   6. A metal-chalcogenide film of any one of the preceding Statements,    where the substrate is a fiber.-   7. A metal-chalcogenide film of any one of the preceding Statements,    where the substrate is silicon, silicon with a silicon oxide layer    or silicon dioxide layer disposed on at least a portion of a surface    of the silicon, quartz, fused silica, mica, silicon nitride, boron    nitride, alumina, or hafnia.-   8. A method of making a metal-chalcogenide film on a substrate, the    film comprising 1 to 10 monolayers of a metal-chalcogenide of any    one of the preceding Statements, comprising: contacting a metal    precursor, a chalcogenide precursor, a reducing gas, and a substrate    in a reactor such that the metal-chalcogenide film is formed (e.g.,    formed by layer-by-layer growth such as, for example, by forming a    first monolayer (e.g., a first monolayer comprising grains that are    connected laterally until the first monolayer is formed across about    80% or greater of the substrate covered by the film) and then,    optionally a second monolayer), where the precursors (metal    precursor and chalcogenide precursor) are each present in a gas    phase and the metal precursor is present at a pressure of 1×10⁻²    Torr or less and the chalcogenide precursor is present at a pressure    of 1×10⁻¹ Torr or less.-   9. A method of making a metal-chalcogenide film on a substrate of    Statement 8, where the contacting is carried out in the presence of    a desiccant.-   10. A method of making a metal-chalcogenide film on a substrate of    any one of Statements 8 or 9, where the contacting is carried out at    a water concentration of less than 1×10⁻² Torr.-   11. A method of making a metal-chalcogenide film on a substrate of    any one of Statements 8 to 10, where the metal precursor has a    concentration lower than a concentration of the chalcogenide    precursor.-   12. A method of making a metal-chalcogenide film on a substrate of    any one of Statements 8 to 11, where partial pressures of the metal    precursor and the chalcogenide precursor are controlled (e.g.,    selected) such that a first monolayer is formed (e.g., grains of a    first monolayer are connected laterally until the first monolayer is    formed across about 80% or greater of the substrate covered by the    film).-   13. A method of forming a metal-chalcogenide film on a substrate of    any one of Statements 8 to 12, further comprising forming a second    monolayer on the first monolayer after the first monolayer is formed    on about 80% or greater of the substrate covered by the film.-   14. A method of forming a metal-chalcogenide film on a substrate of    any one of Statements 8 to 13, where the second monolayer is formed    on the first monolayer after the first monolayer is formed about 95%    or greater of the substrate covered by the film.-   15. A device comprising a metal-chalcogenide film of any one of    Statements 1 to 8 or a metal-chalcogenide film made by a method of    any one of Statements 8 to 14.-   16. A device comprising a metal-chalcogenide film of Statement 11,    where the device is an electronic device.-   17. A device comprising a metal-chalcogenide film of any one of    Statements 15 or 16, wherein the device is a laser, a photo-diode,    an optical modulator, a piezoelectric device, a memory device, or a    thin film transistor.-   18. A device comprising a metal-chalcogenide film of any one of    Statements 15 or 16, where the device is a transistor, P—N junction,    logic circuit, or analog circuit.-   19. A device comprising a metal-chalcogenide film of any one of    Statements 15 or 16, where the device is an optical fiber.-   20. A method of forming a metal-chalcogenide film on a substrate    comprising: providing a metal precursor and a chalcogenide precursor    into a reactor; and contacting the metal precursor and the    chalcogenide precursor to form the metal-chalcogenide film having    one or more monolayers of metal chalcogenide on the substrate, where    the metal precursor and the chalcogenide precursor in gas phases are    provided into the reactor, and wherein partial pressures of the    metal precursor and the chalcogenide precursor are controlled such    that a first monolayer is formed (e.g., grains of a first monolayer    are connected laterally until the first monolayer is formed across    about 80% or greater of the substrate covered by the film).-   21. A method of forming a metal-chalcogenide film on a substrate of    Statement 20, further comprising forming a second monolayer on the    first monolayer after the first monolayer is formed on about 80% or    greater of the substrate covered by the film.-   22. A method of forming a metal-chalcogenide film on a substrate of    any one of Statements 20 or 21, where the second monolayer is formed    on the first monolayer after the first monolayer is formed about 95%    or greater of the substrate covered by the film.-   23. A method of forming a metal-chalcogenide film on a substrate of    any one of Statements 20 to 22, wherein the first monolayer is    continuous.-   24. A method of forming a metal-chalcogenide film on a substrate of    any one of Statements 20 to 23, where the first monolayer has an    area of about 100 square microns or greater.-   25. A method of forming a metal-chalcogenide film on a substrate of    any one of Statements 20 to 24, where the metal precursor has a    concentration lower than a concentration of the chalcogenide    precursor.-   26. A method of forming a metal-chalcogenide film on a substrate of    any one of Statements 20 to 25, further comprising providing a    reducing gas into the reactor.-   27. A method of forming a metal-chalcogenide film on a substrate of    any one of Statements 20 to 26, where a water concentration of the    reactor is controlled using a desiccant.-   28. A method of manufacturing a device comprising the method of any    one of Statements 8 to 14 or 20 to 27.-   29. A method of manufacturing a device comprising the method of any    one of claims 8 to 14 or 20 to 25, where the device is a device of    any one of Statements 15-19.

The following examples are presented to illustrate the presentdisclosure. They are not intended to limiting in any manner.

EXAMPLE 1

This example describes the fabrication and characterization of films ofthe present disclosure.

The growth of semiconducting ML films of MoS₂ and WS₂ on silicon oxideon a 4-inch wafer scale is described, with both desirable electricalperformance and structural continuity, maintained uniformly over theentire films. FIG. 1 shows continuous TMD ML films and shows theirwafer-scale homogeneity and intrinsic optical properties. The photos ofMoS₂ (FIG. 1 a; grey) and WS₂ (FIG. 1 b; light grey) films grown on atransparent 4-inch fused silica wafer show that the TMD grown region(right half) is uniform over the whole substrate and clearlydistinguishable from the bare silica substrate (left half). The opticalabsorption, photoluminescence (PL), and Raman spectra measured from ourfilms show characteristics unique to ML MoS₂ and WS₂, respectively (FIG.1d-f ). All of these measured spectra have the same peak positions as inexfoliated ML samples (denoted by diamonds), regardless of the locationof the measurements within our films (FIG. 5). The X-ray photoelectronspectra (XPS) taken from our ML MoS₂ film show almost identical featuresas in bulk single crystal with low level of defects, further confirmingthe precise chemical composition and the high quality of our MoS₂ film(FIG. 6).

FIG. 1c shows a photo of a MoS₂ film grown on a 4-inch SiO₂/Si wafer.The ML film was patterned using standard photolithography and oxygenplasma etching to form MoS₂ covered squares (dark, 6 mm wide) with anarray of 3 μm holes. A zoomed-in, normalized optical reflection image(FIG. 1h ) displays a homogeneous reflection contrast for the entireMoS₂ covered region, confirming uniform ML growth everywhere with nogaps. In addition, FIG. 1g shows a scanning electron microscope (SEM)image of an array of fully-suspended ML MoS₂ membranes (2 μm indiameter) fabricated by transferring our MOCVD-grown film onto a SiNgrid with holes. Its high fabrication yield (>99.5%) suggests mechanicalstrength and continuity of the film. The widefield PL images of thesefilms (insets, FIGS. 1g and 1h ) show strong, spatially uniform PLsignals, further confirming that they are continuous ML MoS₂, with itshigh quality maintained even after patterning or transfer. The samespatial uniformity was seen in the optical reflection and PL images of aML WS₂ film that was similarly grown and patterned (FIG. 1i ). Together,the data in FIG. 1 confirm that the MoS₂ and WS₂ films are continuousMLs, spatially uniform over the entire 4-inch growth substrates withintrinsic optical properties. Below, using MoS₂ as the main example, thegrowth (FIG. 2) and the excellent electrical properties (FIG. 3) ofthese MOCVD-grown films is discussed.

FIG. 2a schematically explains our MOCVD growth, where gas-phaseprecursors of Mo(CO)₆, W(CO)₆, (C₂H₅)₂S, and H₂, all diluted in Ar as acarrier gas are used. The concentration of each reactant can beprecisely controlled during the entire growth time by regulating thepartial pressure (Px) of each reactant (X). Thus the setup offers adesirable environment for maximizing the areal coverage of the ML, andfor engineering the film structure by controlling the nucleation densityand intergrain stitching. FIG. 2 summarizes the results.

First, the MoS₂ film is grown in the layer-by-layer growth mode, whichis important for the uniform layer control over large scale. FIG. 2cplots the areal coverage of ML (θ_(1L)) and multilayer (θ_(≥2L); mostlybilayer) regions measured from our MoS₂ grown on SiO₂/Si along withoptical images (FIG. 2b ) at different growth times. It shows theinitial nucleation on the SiO₂ surface (t=0.5 t₀), subsequent ML growthnear (0.8 t₀) and at the maximum ML coverage (t₀), followed bynucleation mainly at grain boundaries (1.2 t₀) and bilayer growth (2t₀). Significantly, no nucleation of second layer while the first layeris forming (θ_(≥2L)˜0 when t<t₀) was observed, producing an optimalgrowth time to near full ML coverage (θ_(1L)˜1). Additional PL andelectron microscope images taken after different growth times furthersuggest that the edge attachment is the main mechanism for the ML growthafter nucleation and that the neighboring ML grains are uniformlyconnected by tilt grain boundaries with enhanced PL at t=t₀ (see, FIG.7, FIG. 8). The standard thin film growth model suggests this growthmode is effective below a certain deposition rate of the growth species,above which it suggests a different mode that forms thicker islands.Indeed, the layer-by-layer growth of MoS₂ film was observed only when alow partial pressure (P_(Mo)˜10⁻⁴ Torr in FIGS. 2b and 2c ) of Mo vapour(produced by thermal decomposition of Mo(CO)₆; see FIG. 9) was appliedunder the presence of excess (C₂H₅)₂S. In contrast, the growth at ahigher P_(Mo) is no longer in the layer-by-layer growth mode, insteadproducing a mixture of ML, multilayer, and no growth regionssimultaneously (FIG. 10). For the uniform ML growth over a largesubstrate, it is thus important to maintain a low P_(Mo) constantly overthe entire growth region and over time, the key technical capabilityprovided by our MOCVD setup (see FIG. 11 for the spatially homogeneousML nucleation over multi-inch scale).

Second, the grain structure of our MoS₂ film, including the averagegrain size and the intergrain connection, depends sensitively on theconcentrations of H₂, (C₂H₅)₂S as well as residual water. As arepresentative example, FIG. 2d shows the two main effects of H₂, whosepresence is necessary for removing carbonaceous species generated duringthe MOCVD growth: (i) the average grain size increases from hundreds ofnm to over 10 μm with decreasing H₂ flow, and (ii) the MoS₂ grains grownunder higher H₂ flow (FIG. 2d , right image) have mostly perfecttriangular shapes without merging with neighbouring grains, a trend thatdisappears with lower H₂ flow (left and middle images). Theseobservations are consistent with the H₂ induced decomposition of(C₂H₅)₂S (increasing nucleation due to hydrogenolysis), and the etchingof the MoS₂ (preventing intergrain connection) as previously reported(for further discussion on the effects of (C₂H₅)₂S and water, see FIG.10). In order to grow continuous ML MoS₂ with large grain size andhigh-quality intergrain stitching, we thus flow optimal amounts of H₂and (C₂H₅)₂S and dehydrate the growth environment.

The darkfield transmission electron microscope (DF-TEM) and annulardarkfield scanning TEM (ADF-STEM) images shown in FIGS. 2e and 2fconfirm the structural continuity of our MoS₂ film grown under thoseconditions on the nanometre and atomic length scales. The DF-TEM imageshows a continuous polycrystalline ML film with no visible gaps, whilehaving less than 0.5% bilayer area. Further analysis of the DF-TEM andelectron diffraction data (see FIG. 12) confirms a uniform angulardistribution of crystal orientations with no preferred intergrain tiltangle for grain boundaries. The ADF-STEM data (FIG. 2f , more imagesshown in FIG. 13) further confirm that adjacent grains are likely to beconnected by a high quality lateral connection with similar structuresseen in previous reports. The MoS₂ films shown in FIG. 1 as well as theones studied below for their electrical properties in FIG. 3 and FIG. 4were grown, producing an average grain size of ˜1 μm (see FIGS. 2b and2e ). Surprisingly, almost identical growth parameters with P_(W) ˜10⁻⁴Torr produce ML WS₂ films as shown in FIGS. 1b and 1 i, indicating thesame layer-by-layer growth for WS₂ with a similar t₀.

The electrical properties of our ML MoS₂ films display two importantcharacteristics: the spatial uniformity over a large scale and excellenttransport properties similar to those seen in exfoliated samples. Allour electrical measurements in FIG. 3 and FIG. 4 (except for FIG. 3c )were performed at room temperature. FIG. 3a first shows the sheetconductance (σ_(□)) vs backgate voltage (V_(BG)) curves measured from aML MoS₂ FET (optical image shown in the inset) with multiple electrodesfor the four-probe measurements (except for channel length (L)=34 μm).It includes several curves for different L ranging between 1.6 and 34 μm(shifted from the bottom one for clarity), all of which show nearlyidentical behaviours, including the n-type conductance, carrierconcentration (˜4×10¹² cm⁻² at V_(BG)=0 V), and high field effectmobility (μ_(FE)). FIG. 3b further plots μ_(FE) measured from five suchdevices, fabricated at random locations and separated by up to 3.3 mm ona single chip. All the devices show similar μ_(FE) near 30 cm²/Vsindependent of L and device location with similarly uniform σ_(□)-V_(BG)curves (shown in FIG. 14), suggesting the spatial homogeneity of theelectrical properties of the MoS₂ film at length scales ranging frommicrometres to millimetres.

The distribution of μ_(FE) of the devices is compared with the resultsof multiple devices from two previous reports, each measured fromindividual grains of exfoliated or CVD grown MoS₂ samples. Surprisingly,μ_(FE) measured from our MOCVD film is similar to the median μ_(FE)(denoted by a star) of exfoliated samples (and several times higher thanthe CVD results), while displaying a much narrower distribution. Inaddition, the temperature dependence of μ_(FE) (FIG. 3c ) measured fromthe same device in FIG. 3a shows higher μ_(FE) at lower temperatures (92cm²/Vs at 100K) and the intrinsic, phonon-limited electron transport,similar to the behaviours previously observed in exfoliated samples(shown in FIG. 3c ) but different from those observed from a CVD samplewith stronger effects from defects. Specifically, our data show thecharacteristic of μ_(FE) ˜T^(−γ) dependence between 150 and 300K withγ=1.6, close to the value predicted by theory (1.69) and consistent withresults from previous experiments (average value ranging between 0.6 and1.7) for a similar temperature range. Finally, FIG. 3d shows ahigh-performance MoS₂ FET fabricated with an individual top-gateelectrode (V_(TG)). It has a high on/off conductance ratio (˜10⁶),current saturation at relatively low bias V_(SD) (lower inset, FIG. 3d), high field effect mobility (˜29 cm²/Vs) and large transconductance(˜2 μS/μm), all of which are comparable to the best reported results.The devices studied in FIG. 3a-3d were fabricated at random locationsusing a polycrystalline ML MoS₂ film, unlike the devices withsingle-grain samples used for comparison. In addition, the electricalproperties measured from a separate ML MoS₂ film with a larger averagegrain size of 3 μm (instead of 1 μm in FIG. 3) show almost identicalcharacteristics, including the channel length independence of μ_(FE) andthe phonon-limited transport at T>150K (see FIG. 15; with the lowtemperature mobility as high as 114 cm²/Vs at 90K). Altogether, our dataconfirm the spatial uniformity and high electrical performance of ourMoS₂ FETs independent of the average grain size, which suggests that theinter-grain boundaries in our film do not significantly degrade theirelectrical transport properties. This is likely due to the formation ofwell-stitched inter-grain boundaries with a low level of defects, anexplanation also supported by the ADF-STEM (FIG. 2f ) and XPS data (FIG.6) discussed earlier. Therefore, our data lead to an importantconclusion that our optimized MOCVD growth provides an electricallyhomogeneous ML MoS₂ film. Moreover, we successfully fabricated andmeasured 60 FETs using a ML WS₂ film. Even though the growth of ML WS₂is not carefully optimized, these devices show excellent electricalproperties with their μ_(FE) as high as 18 cm²/Vs at room temperature(FIG. 3e ) with the median μ_(FE) close to 5 cm²/Vs. In addition, theWS₂ device in FIG. 3e shows a high on/off ratio of 10⁶ and the currentsaturation behaviour (inset, FIG. 3e ) as in our MoS₂ devices. (FIG. 16for data from additional ML WS₂ FET devices).

The structural and electrical uniformity of our MoS₂ film enables thewafer-scale batch fabrication of high performance FETs as demonstratedin FIGS. 3f and 3g . FIG. 3f shows a photo of 8,100 MoS₂ FETs with aglobal back gate, which were fabricated on a 4-inch SiO₂/Si wafer usinga standard photolithography process. FIG. 3f -2 (3 f-3) shows thecolour-scale map of σ_(□) measured from 100 MoS₂ FETs in one squareregion at V_(BG)=+50 V (−50 V) along with the zoomed-in optical image ofthe devices (FIG. 3f -1). An almost perfect device yield of 99% wasobserved, only two out of 200 FETs we characterized (including data froman adjacent region) do not conduct. The data also confirm the spatiallyuniform n-type transistor operation (larger at for positive V_(BG)) withsimilar V_(BG) dependence for all our devices and high on-state deviceconductance. Similarly uniform V_(BG) dependence from FET devicesfabricated using ML MoS₂ films with different average grain sizes wasalso observed, as characterized by the histograms of the thresholdvoltages (FIG. 17). Similarly, 100 individually addressable dual gateMoS₂ FETs (similar to the device in FIG. 3d ) were fabricated on anotherwafer piece. The histogram of the on-state σ_(□) (V_(TG)=5 V; mediancarrier concentration ˜7×10¹² cm⁻²) and off-state σ_(□) (V_(TG)=−5 V)collected from all such FETs (FIG. 3g ) shows strong peaks above 10⁻⁵ Sand near 10⁻¹¹ S, respectively, confirming a uniform conductanceswitching behaviour with high on-state σ_(□) (>10 μS) and on-off ratio(˜10⁶). In addition, the majority of these batch-fabricated FETs showshigh μ_(FE) (>10 cm²/Vs, see FIG. 18).

The data presented in FIG. 1-3 confirm the structural and electricaluniformity of the wafer-scale ML MoS₂ film grown by the instant MOCVDmethod. This makes the film compatible with the batch device fabricationprocesses on a technologically relevant scale. Moreover, as SiO₂provides a substrate for its growth, one can produce high quality MLfilms on a variety of substrates by depositing SiO₂ prior to the growth.This versatility would allow the realization of high performance FETsdirectly on non-conventional substrates, such as metal and thermallystable plastic. In addition, one can integrate multiple layers of MoS₂devices by repeating the TMD film growth, device fabrication, and SiO₂deposition, which could enable novel three-dimensional circuitry.

In FIG. 4, this potential was demonstrated by producing multi-stacked MLMoS₂ films as well as electronic devices fabricated at differentvertical levels. FIG. 4a shows the schematics and photos of threesubstrates each with single, double or triple ML MoS₂ films grown atdifferent levels. The first (bottom) ML film was grown on a fused silicasubstrate while the additional layers were grown on SiO₂ (100 nm thick)deposited on the previously grown MoS₂ ML using plasma-enhanced CVD(PECVD). The colour of the substrate, which remains uniform for eachsubstrate, becomes darker as the number of layers increases. Theirabsorption spectra, shown in FIG. 4b , present almost identicalabsorption at all measured wavelengths, once normalized by the number ofstacks grown (see inset), suggesting little degradation of the opticalproperties of the ML MoS₂ films after subsequent oxide deposition andMoS₂ growth.

FIG. 4c shows the schematics of our multi-stacked device fabricationprocess: (i) first ML MoS₂ growth on a SiO₂/Si wafer, (ii) FETfabrication, (iii) deposition of SiO₂ (thickness of 500 nm), (iv) secondML MoS₂ growth and FET fabrication. A false-colour SEM image in FIG. 4dshows an array of MoS₂ FETs successfully fabricated using this process.It includes functioning MoS₂ FETs located at two different verticallevels, the conductance of which can be simultaneously modulated with aglobal back gate. The I_(SD)-V_(SD) curves measured from two adjacentFETs located next to each other, both laterally and vertically (seeinset, FIG. 4d ), are shown in FIG. 4e . Both devices show V_(BG)dependent conductance change (notice the smaller change for the 2^(nd)layer) with an on-state σ_(□) of 2.5 μS (1^(st) layer) and 1.5 μS(2^(nd) layer), respectively. Furthermore, similar μ_(FE) values (11.5cm²/Vs and 8.8 cm²/Vs) from the two devices (FIG. 19) were measured. Thetwo ML MoS₂ films were grown on SiO₂ substrates prepared differently andthat the 1^(st) layer device has gone through additional steps,including the second MoS₂ growth. The data in FIG. 4 thus confirm thecompatibility of the instant MOCVD-grown MoS₂ films with conventionalthin film deposition and multi-stacking, which could be used to developa three-dimensional device architecture based on TMD.

The high-mobility ML TMD films can be immediately utilized for the batchfabrication of TMD-based integrated circuitry consisting of FETs,photodetectors and light emitting diodes, on a technologically-relevantmulti-inch wafer scale. In addition, as the MOCVD growth is controlledby the kinetics of precursor supply rather than specificprecursor/substrate chemistry (an example of the latter would be thedifferent graphene growth modes on Cu vs Ni), its use is not limited tothe TMD/substrate combinations reported here. Instead, it could begeneralized for producing various TMD materials, both semiconductor(e.g. MoSe2, WTe₂) and metal (e.g. NbSe₂, TaS₂), with precise layercontrol over a large scale. Indeed, the data show that, as an initialdemonstration, the ML TMD growth is possible on a variety of othertechnologically important substrates (Al₂O₃, SiN, HfO₂) with the samegrowth conditions developed for SiO₂ (see FIG. 20 for MoS₂ growth anddevice fabrication on these substrates using these unoptimizedconditions). Therefore, the versatile MOCVD growth provides an excitingnew avenue for the growth, patterning and integration of multiple,high-quality ML TMD films with different compositions and electricalproperties on a single substrate, enabling the future development ofatomically thin integrated circuitry.

MOCVD growth of ML MoS₂ and WS₂ films. As illustrated in FIG. 2a , thesynthesis of ML MoS₂ and WS₂ was carried out in a 4.3-inch (innerdiameter) hot-wall quartz tube furnace. Molybdenum hexacarbonyl (MEW),tungsten hexacarbonyl (THC), diethyl sulphide (DES), which have highequilibrium vapour pressure near room temperature, are selected aschemical precursors for Mo, W, S, respectively, and introduced to thefurnace in gas phase. H₂ and Ar are injected to the chamber usingseparate lines. All the precursors used in our MOCVD growth arecommercially available with well-documented safety protocols (MEW: SigmaAldrich 577766, THC: Sigma Aldrich 472956, DES: Sigma Aldrich 107247).The safety ratings for these precursors require them to be handledinside of a fume hood. (MHC, THC: NFPA rating for health hazard 4, DES:NFPA rating for health hazard 2). The optimum growth parameters for MLMoS₂ and WS₂ films are as follows. We use a total pressure of 7.5 Torr,growth temperature of 550° C. and growth time (t₀) of 26 hrs. The flowrate of precursors are 0.01 sccm for MEW or THC, 0.4 sccm for DES, 5sccm for H₂, and 150 sccm for Ar, which were regulated by individualmass flow controllers (MFCs). The low flow rates were used for MEW, THCand DES for the layer-by-layer growth mode. The long growth time (t₀˜26hrs), is necessary for full ML growth, because of the low growth rate inthis regime. NaCl is loaded in the upstream region of the furnace as adesiccant to dehydrate the growth chamber, which significantly increasesthe grain size, as discussed in FIG. 10. We use a 4-inch fused silicawafer or a 4-inch Si wafer with 285 nm thick thermal SiO₂ as the maingrowth substrates. Also the growth is possible on Al₂O₃, HfO₂, and SiN(see FIG. 20).

Optical measurements. Film patterning: Photolithography was performed tomake the hole-array pattern on the MoS₂ film, where a sacrificial layerof PMMA A4 is coated before the photoresist. O₂ plasma (400 W, 300 s)was used to remove the unwanted MoS₂ and sacrificial PMMA from the SiO₂surface. The chips were then placed in acetone for 3 hours to thoroughlyremove the photoresist and the PMMA residue.

Optical absorption: The absorption measurements were performed with aShimadzu UV-Vis-NIR Spectrometer under ambient conditions. All measuredsamples were grown on a fused silica substrate, and a bare fused silicasubstrate was used as the reference.

Photoluminescence: The photoluminescence (PL) measurements wereperformed with a 532 nm excitation laser under ambient conditions. ThePL spectra from the sample were collected by an imaging spectrometerwith a CCD camera, and the PL images were taken directly using band passfilters with the centre wavelength corresponding to 1.9 eV for MoS₂ and2.0 eV for WS₂.

TEM analysis. Sample preparation: The ML MoS₂ film grown on a SiO₂/Sisubstrate was coated by PMMA A2 or A4, and then the substrate was etchedin KOH 1M solution at 90° C. After being rinsed in deionized water threetimes, the PMMA supported MoS₂ film was transferred to a TEM grid, andthe chip was annealed in an ultra-high vacuum (10⁻⁷ Torr) or atmosphericpressure with Ar (100 sccm) and H₂ (100 sccm) flow at 350° C. for 3hours in order to remove the PMMA.

DF-TEM: DF-TEM images, along with electron diffraction patterns, weretaken using an FEI Tecnai T12 Spirit, operated at 80 keV. Theacquisition time for each dark field image was 10 seconds.

ADF-STEM: ADF-STEM images were taken using a Nion Ultra STEM 100operated at 60 keV. The convergence angle was 30 mrad, and the probecurrent was about 50 pA.

Device fabrication. For the FET fabrication, the process was startedwith an as-grown ML TMD film on 285 nm SiO₂/Si and first define thesource and drain electrodes using the standard photolithography process,followed by e-beam evaporation of 0.5 nm Ti/75 nm Au. After lifting offusing Microposit Remover 1165, the conducting channel for FET deviceswere defined and etched using photolithography and O₂ plasma etching.For top gate fabrication, 30 nm HfO₂ is deposited using atomic layerdeposition (ALD) as the dielectric material, followed by the sameelectrode fabrication process for top-gate electrode fabrication (fortop gate WS₂ FETs, we deposit 1 nm Al₂O₃ as the seeding layer for HfO₂ALD). 30 nm HfO₂ was deposited on top of the back gated devices. Thisincreases the carrier doping level and the conductance of our devices,enabling reproducible measurements under ambient conditions (see FIG.21). All devices shown in FIGS. 3 and 4 were fabricated using standardphotolithography techniques, except for the additional voltage probes(the five thin electrodes in FIG. 3a , inset) which were added laterusing e-beam lithography. The dimensions (W, width and L, length) ofconducting channels are as follows: W 15 μm (FIG. 3a-3c ), W 9 μm, L 19μm (FIGS. 3d and 3g ), W 7.7 μm , L 3.3 μm (FIG. 3e ), W 7.7 μm, L 5.3μm (FIG. 3f ), W 15 μm, L 15 μm (FIGS. 4d and 4 e). For the fabricationof the multi-stacked device in FIG. 4, the SiO₂ consists of threesuccessive depositions: 100 nm SiO₂ was deposited using PECVD at 30 Wand 200° C., followed by 350 nm SiO₂ deposited using PECVD at 140 W and350° C., and 50 nm SiO₂ deposited using ALD at 200° C.

Electrical measurements. All the electrical measurements (except forFIG. 3c ) were done under ambient conditions using a custom-built probestation with an Agilent B1500 Device Analyzer. Both four-probe andtwo-probe measurements were used to accurately measure the sheetconductance. Comparing the results of four-probe and two-probemeasurements, the contact resistivity is estimated to be approximately50 Ω·mm. For temperature dependent measurements (FIG. 3c ), FET deviceswere wirebonded and measured in a cryostat in vacuum for temperaturesdown to 77K.

A. Growth mechanism of layer-by-layer (LBL) growth. Two experiments wereconducted to support LBL growth mechanism. First, FIG. 7 presentsoptical reflection (OR), PL and SEM images taken from our MOCVD grown MLMoS₂ films (average grain size ˜3 μm) after different growth times (0.8t₀, 1.0 t₀, and 1.5 t₀). It confirms that additional nucleation on theexisting first layer does not occur until the first layer growth iscompleted and the growth proceeds by enlarging the already-nucleatedgrains for t<t₀, most likely by edge attachment growth. After t₀ (fullfirst layer growth), the nucleation for the second layer occurs at thegrain boundaries (GBs) and in the basal plane. The PL intensity imagesshow striking behaviours especially at the GBs. They show much brighterPL uniformly along GBs at t=t₀ and much darker PL there when t>t₀. Thebrighter PL at t=t₀ is consistent with the PL behaviours previously seenfrom the tilt GBs in CVD grown MoS₂. This suggests that upon thecompletion of the first layer growth (t=t₀), neighbouring grains areuniformly connected laterally by tilt GBs before further growth occursfor the second layer. Once the second layer starts growing on top of thefirst layer, the PL signal decreases especially along GBs, as the bandstructure of MoS₂ shifts from direct bandgap (ML) to indirect bandgap(for multilayers). Altogether, our data in FIG. 7 confirm the LBL growthmode in the MOCVD growth and the high-quality uniform intergrainconnection at the optimum growth time (t₀). We also note that monitoringPL enhancement along GBs can be used in the future to find the optimumgrowth time t₀ and to confirm the high quality intergrain connection inthe grown film.

The edge attachment growth mechanism for our MOCVD growth is furthersupported by FIG. 8. Here, we performed a partial growth of ML MoS₂(step I; t<t₀) and re-growth (step II), where the step II was conductedon the same sample generated after step I. The same film wascharacterized by optical reflection, PL, DF-TEM after each growth stepto observe the location and morphologies of the growth. The data confirmthat during the re-growth, MoS₂ grains continue to grow by edgeattachment without generating additional nucleation sites on availableSiO₂ surfaces. This is confirmed by the same average number of nuclei(or grains in the first layer) per unit area after step I and step II(0.27 μm⁻²→4 0.26 μm⁻²), again indicating that the empty space afterstep I was completely filled by continued growth of existing grains(along their edges) with the same crystal orientation without creatingadditional grains.

B. Thermal decomposition of precursors. We studied thermal reaction forDES and MHC using a residual gas analyser (RGA), which connected to theoutlet of the furnace and detects the mass signal of the gas residue.FIG. 9 shows the relative intensity ratio for corresponding moleculesextracted from mass spectra of RGA. First, we flowed vaporized MHC intothe chamber at room temperature (Room T), and we confirmed thatvaporized MHC contains several carbonyl molybdenum, Mo(CO)_(x). Above250° C. the signal for Mo(CO)_(x) disappeared, indicating thatMo(CO)_(x) was completely decomposed. In the case of DES, the intensityprofiles at room temperature are almost the same as at growthtemperature, 550° C. (Growth T), with both showing various hydrocarbonsulphides (C_(x)H_(y)S) under RGA resolution. This means that theconcentration of DES in the furnace barely changed due to itsdecomposition. According to the RGA study, we summarize the status ofprecursors at growth temperature: i) the concentration of C_(x)H_(y)S isuniform inside the furnace at the laminar flow condition. ii) MHC isdecomposed to Mo and delivered by high flow Ar.

C. Dependence of grain size on concentration of H₂, H₂O, and DES. Wehave already shown the H₂ concentration dependence of grain size in FIG.2d . The dependence on H₂O concentration was observed under the presenceof salt desiccant (NaCl, KCl, NaBr), as shown in FIG. 10a , where thegrain size increased up to 100-fold, according to the presence/absenceof salt. Also, FIG. 10b shows that the concentration of DES affects thegrain size.

In order to explain these phenomena, we need to discuss the precursordecomposition and nucleation kinetics. First, according to hydrolysisand hydrogenolysis, H₂ and H₂O promote the decomposition of DESprecursor, which enhances the concentration of sulphur vapour. Also, theconcentration of sulphur vapour linearly depends on the concentration ofDES, since DES contains certain ratio of sulphur vapour. Second, theconcentration of sulphur affects the nucleation kinetics and grain size.The assumptions we make are: (i) our growth is Mo diffusion limitedgrowth, since the Mo concentration is kept low for layer-by-layergrowth. In comparison, the concentration of DES is much higher than thatof Mo vapour. (ii) when a Mo atom produced by thermal decomposition ofMHC, arrives at the surface, it diffuses until reacting with sulphurproduced by decomposition of DES. (iii) energetically, Mo and S atomsprefer to be adsorbed at a MoS₂ edge. (iv) if the decomposition rate ofDES is fast, Mo atoms lose their chance to find energetically favourablepositions and nucleation occurs at a non-edge region. Based on theseassumptions, we conclude that the nucleation density of MoS₂ increaseson the surface when the decomposition kinetics of DES becomes faster.Therefore, when H₂, H₂O, and DES concentrations are high, nucleationdensity increases and grain size decreases.

Although the present disclosure has been described with respect to oneor more particular embodiments and/or examples, it will be understoodthat other embodiments and/or examples of the present disclosure may bemade without departing from the scope of the present disclosure.

1. A metal-chalcogenide film disposed on a substrate, the filmcomprising a monolayer of a metal-chalcogenide.
 2. Themetal-chalcogenide film of claim 1, wherein the metal is Sc, Ti, V, Cr,Mn, Fe, Co, Ni, Nb, Mo, W, or a combination thereof.
 3. Themetal-chalcogenide film of claim 1, wherein the chalcogenide is sulfide,selenide, telluride, or a combination thereof.
 4. The metal-chalcogenidefilm of claim 1, wherein the film comprises 2 to 10 monolayers of ametal chalcogenide.
 5. The metal-chalcogenide film of claim 1, whereinthe film is structurally and electrically continuous.
 6. Themetal-chalcogenide film of claim 1, wherein the film is continuous over95% or greater of the substrate that is covered by the film.
 7. Themetal-chalcogenide film of claim 1, wherein the substrate is a fiber. 8.The metal-chalcogenide film of claim 1, wherein the substrate issilicon, silicon with a silicon oxide layer or silicon dioxide layerdisposed on at least a portion of a surface of the silicon, quartz,fused silica, mica, silicon nitride, boron nitride, alumina, or hafnia.9. A method of making a metal-chalcogenide film on a substrate, the filmcomprising 1 to 10 monolayers of the metal-chalcogenide comprising:contacting a metal precursor, a chalcogenide precursor, a reducing gas,and a substrate in a reactor such that the metal-chalcogenide film isformed, wherein the metal precursor and the metal chalcogenide precursorare each present in a gas phase and the metal precursor is present at apressure of 1×10⁻² Torr or less and the chalcogenide precursor ispresent at a pressure of 1×10⁻¹ Torr or less.
 10. The method of claim 9,wherein the contacting is carried out in the presence of a desiccant.11. The method of claim 9, wherein the contacting is carried out at awater concentration of less than 1×10⁻² Torr.
 12. The method of claim 9,wherein the metal precursor has a concentration lower than aconcentration of the chalcogenide precursor.
 13. The method of claim 9,wherein partial pressures of the metal precursor and the chalcogenideprecursor are controlled such that grains of a first monolayer areconnected laterally until the first monolayer is formed across about 80%or greater of the substrate covered by the film.
 14. The method of claim12, further comprising forming a second monolayer on the first monolayerafter the first monolayer is formed on about 80% or greater of thesubstrate covered by the film.
 15. The method of claim 13, wherein thesecond monolayer is formed on the first monolayer after the firstmonolayer is formed on about 95% or greater of the substrate covered bythe film.
 16. A device comprising a metal-chalcogenide film of claim 1.17. The device of claim 16, wherein the device is an electronic device.18. The device of claim 17, wherein the device is a laser, aphoto-diode, an optical modulator, a piezoelectric device, a memorydevice, or a thin film transistor.
 19. The device of claim 17, whereinthe device is a transistor, P—N junction, logic circuit, or analogcircuit.
 20. The device of claim 16, wherein the device is an opticalfiber.